The choice of 700 or 800 MHz has nothing to do with simulcast vs single site, it's just what was available for licensing at the time the sites were built.
Most of the new sites use 700 MHz since there were no 800 MHz pairs free in the area or they were tied up in the rebanding freeze. The few 800 MHz sites obtained their channels by reuse of frequencies from the older systems they replaced.
There's no technical restriction on mixing 700 & 800 MHz frequencies in the same site, other than having an antenna system to handle it. So in the future you could see sites with both, especially after the old MARCS system is shut down and the old 800 MHz frequencies are used to add capacity to MARCS-IP.